Integrated circuits (ICs) include a multitude of transistors formed on a semiconductor substrate. Various methods of forming transistors are known in the art. Generally, transistors are isolated from each other by insulating or isolation structures.
One method of forming transistors on a silicon substrate involves the well-known Local Oxidation of Silicon (LOCOS) process. A conventional LOCOS process typically includes the following simplified steps. First, a silicon nitride layer is thermally grown on the silicon substrate. Generally, conventional LOCOS processes require a high quality, thermally grown silicon nitride layer to avoid examination and other processing problems. Next, using a lithography and etch process, the nitride layer is selectively removed to produce a pattern where transistor source/drain areas are to be located. After patterning the source/drain areas, a field oxide is grown. As oxide growth is inhibited where the nitride layer still remains, the field oxide only grows on the silicon substrate exposed during the source/drain patterning step. Finally, after oxide growth is complete, the remaining portions of the nitride layer are removed, leaving only the oxidized source/drain areas on the exposed silicon substrate.
Another process for forming insulating structures and defining source and drain regions is a shallow trench isolation (STI) process. A conventional STI process typically includes the following simplified steps. First, a silicon nitride layer is thermally grown or deposited onto the silicon substrate. Next, using a lithography and etch process, the silicon nitride layer is selectively removed to produce a pattern where transistor source/drain areas are to be located. After patterning the source/drain areas, the substrate is etched to form trenches. After the trenches are formed, a liner is thermally grown on the exposed surfaces of the trench. The liner oxide is typically formed at a very high temperature in a hydrochloric (HCl) acid ambient. An insulative material such as silicon dioxide (SiO2) is blanket deposited over the nitride layer and the liner oxide within the trench. The insulative material is polished to create a planar surface. The nitride layer is subsequently removed to leave the oxide structures within the trenches.
Shallow trench isolation (STI) structures are utilized in strained silicon (SMOS) processes. SMOS processes are utilized to increase transistor (MOSFET) performance by increasing the carrier mobility of silicon, thereby reducing resistance and power consumption and increasing drive current, frequency response, and operating speed. Strained silicon is typically formed by growing a layer of silicon on a silicon-germanium substrate or layer.
The silicon-germanium lattice associated with the silicon-germanium substrate is generally more widely spaced than a pure silicon lattice, with spacing becoming wider with a higher percentage of germanium. Because the silicon lattice aligns with the larger silicon-germanium lattice, a tensile strain is created in the silicon layer. The silicon atoms are essentially pulled apart from one another.
Relaxed or non-strained silicon has a conductive band that contains six equal valence bands. The application of tensile strain to the silicon causes four of the valence bands to increase in energy and two of the valence bands to decrease in energy. As a result of quantum effects, electrons effectively weigh 30 percent less when passing through the lower energy bands. Thus, the lower energy bands offer less resistance to electron flow. In addition, electrons meet with less vibrational energy from the nucleus of the silicon atom, which causes them to scatter at a rate of 500 to 1000 times less than in relaxed silicon. As a result, carrier mobility is dramatically increased in strained silicon compared to relaxed silicon, providing an increase in mobility of 80% or more for electrons and 20% or more for holes. The increase in mobility has been found to persist for current fields up to 1.5 megavolts/centimeter. These factors are believed to enable a device speed increase of 35% without further reduction of device size, or a 25% reduction in power consumption without a reduction in performance.
In conventional SMOS processes, a strained silicon layer is provided over a silicon-germanium layer and etched to form trenches for the STI structure. The trenches extend through the strained silicon layer and at least partially into the silicon-germanium layer. A conventional STI liner oxide process is utilized to grow a trench liner. The conventional STI liner oxide process can utilize very high temperatures and HCl ambient environments. During this process, germanium outgassing is enhanced, which can negatively impact the formation of thin films, can contaminate IC structures, layers and equipment, and can cause germanium accumulation or “pile-up” at the interface of the liner, thereby causing reliability issues for the STI structure.
In conventional SMOS processes, the STI structure can be subject to silicon overhang at the edge of the STI structure because the silicon-germanium layer is more readily etched than the strained silicon layer. Further, the strained silicon layer is more easily consumed than the silicon-germanium layer during trench liner and filling processes. In addition, conventional SMOS STI trenches can be subject to STI sidewall leakage.
Thus, there is a need for an STI structure which can be formed without damaging strained materials or layers. Further still, there is a need for a process of forming high quality oxides with good compatibility that are not susceptible to problems associated with strained materials. Further still, there is a need for an improved SMOS trench formation process that is not susceptible to silicon overhang. Yet further, there is a need for a liner formation process that is less susceptible to germanium outgassing. Further, there is a need for an STI process that is less susceptible to at least one of strained silicon consummation, STI sidewall leakage, and silicon overhang.